1. Field of the Invention
The invention is directed to systems and methods of promoting endothelialization of a hybrid hemodialysis access graft or a femoral artery bypass graft in a mammal.
2. Background of the Related Art
Hemodynamics plays an obligate role on the function and phenotype of vascular cells (i.e. endothelial cells, smooth muscle cells, fibroblasts, etc.) and tissues in the cardiovascular system during disease and healthy states. Cardiovascular disease is the leading cause of death in North America, Europe and the developing world, with coronary heart disease and atherosclerosis being amongst the most prominent cardiovascular diseases. Atherosclerosis is a disorder in which the coronary arteries become clogged by the build up of plaque along the interior walls of the arteries, leading to decreased blood flow which can in turn cause hypertension, ischemias, strokes and, potentially, death. Associated systemic risk factors include hypertension, diabetes mellitus, and hyperlipidemia, among other factors.
Atherosclerosis and other cardiovascular diseases, such as peripheral arterial disease (PAD), occur regularly and predictably at sites of complex hemodynamic behavior and, consequently, motivates further investigation into the role of hemodynamics in cardiovascular diseases. For example, atherosclerosis has been shown to occur in sites of complex hemodynamic behavior. Surgical intervention is often employed to treat it, and may include insertion of a balloon catherter to clean out the plaquie, and insertion of a stent within the vessel to enable it to remain open, or may include multiple bypasses of the clogged vessels. Bypass surgery involves the removal of a section of vein from the patient's lower leg, and its transplant into the appropriate cardiac blood vessels so that blood flows through the transplanted vein and thus bypasses the clogged vessels. A major problem associated with bypass surgery is the patency of the vessels to be used in the bypass. The bypass vessels are prone to failure, which may occur within a short period of time after bypass surgery, or after a period of several years. Hemodynamic forces have been implicated as a major factor contributing to the failure of the bypass vessels.
Hemodynamic forces, which are forces generated by irregular flow, and in particular, by the (sometimes irregular) flow of blood, are known to have numerous influences on blood vessels, including, but not limited to effects on blood vessel cell structure, pathology, function, and development. In the specific example of blood vessel structure and pathology, the vascular cells lining all blood vessels, endothelial cells (ECs), are important sensors and transducers of two of the major hemodynamic forces to which they are exposed. These forces include wall shear stress (“WSS”), which is the fluid frictional force per unit of surface area, and hoop stress, which is driven by the circumferential strain (“CS”) of pressure changes. Wall shear stress acts along the blood vessel's longitudinal axis, while circumferential strain is associated with the deformation of the elastic artery wall (i.e., changes in the diameter of the vessel) in response to oscillation or variation in vascular pressure. Wave reflections in the circulation and the inertial effects of blood flow cause a phase difference, the stress phase angle (“SPA”), between CS and WSS. The SPA vanes significantly throughout the circulation, and is most negative in disease prone locations, such as the outer walls of a blood vessel bifurcation such as the carotid sinus and the coronary arteries. Hemodynamic forces have been shown to dramatically alter endothelial cell function and phenotype (i.e., higher shear stress [low SPA] is associated with an atheroprotective gene expression profile, and a low shear stress [large SPA] is associated with an atherogenic gene expression profile).
ECs can influence vasoactivity and cause vessels to contract or dilate depending on the blood flow (shear stress) and pressure (causing stretch or CS), and thus are one component which is critical to blood pressure regulation among the many important factors which influence and/or are dependent on the hemodynamics. ECs are just one type of cell which is directly influenced by hemodynamics. Numerous other cell types may also directly or indirectly influenced by hemodynamics and mechanical forces.
As discussed above, hemodynamic forces have been shown to dramatically alter endothelial function and phenotype. For example, the coronary arteries are the most disease prone arteries in the circulation and have the most extreme SPA in the circulatory system, typically having a large, negative value, yet do not have a particularly low shear stress magnitude, thus suggesting that complex hemodynamic factors that include the SPA are important in cardiovascular function and pathology. Accordingly, there is a great need to study vascular biology in a complete, integrated, and controlled hemodynamic environment, preferably in 3-dimensions. However, to date, detailed knowledge of the simultaneous, combined influence of the time varying patterns of WSS and CS on EC biological response has not been technologically feasible.
More specifically, existing systems have focused on the individual effects of either WSS or strain on ECs separately. The most common WSS systems use a 2-dimensional stiff surface, such as, for example, a glass slide, for the EC culture on the wall of a parallel plate flow chamber, or a cone-and-plate type chamber, to simulate wall shear stress alone, which is only one hemodynamic condition. In such a system, the WSS must usually remain steady due to difficulties in simulating pulsatile flow, and strain or stretch effects must be omitted. Further, cyclic straining devices can only generate strain by stretching cells on a compliant membrane, without flow, and typically only in 2-dimensions. Both types of systems are obviously limited in the fidelity with which they can simulate a true, complete hemodynamic environment.
To address the need for simultaneous pulsatile strain and shear stress, a silicone tube coated with ECs was introduced. However, simulators using these tubes could only achieve phase angles (SPA) of about −90 degrees, if any, which is inadequate for simulating coronary arteries (SPA>−180 or −250 degrees), the most disease prone vessels in the circulation, or other regions of the circulation such as peripheral circulation, carotid, renal, organ hemodynamics, or head and brain hemodynamics, to name a few. A more complete physiologic environment which provides time-varying uniform cyclic CS and pulsatile WSS in a 3-dimensional configuration over a complete range of SPA is sill needed.
Substantially all past research and development has focused only on obvious, one-dimensional blood flow or shear stress hemodynamic force characteristics, even though, based on physics, mathematics, and experimentation, there are clearly a multitude of dimensions associated with the with many simultaneous hemodynamic forces present in vivo, such as pressure and strain. Physiologic environments are highly dynamic and nonlinear, the cardiovascular system is certainly no exception. There is a need to preserve 3-dimensional vascular geometry while simultaneously and independently controlling hemodynamic forces such as, for example, pressure, flow, and stretch, as well as many other parameters and forces) in a cell and tissue culture environment in order to more fully and more accurately recapitulate in vivo hemodynamic environments.